1. Field of the Invention
The invention relates to a lifting gear valve arrangement for controlling a double-action lifting gear or an add-on unit of a mobile implement.
2. Description of Related Art
In modern tractors of the medium and upper performance classes, electrically controllable directional control valves have increasingly been used in work hydraulics for controlling the work functions of coupled devices. The control of these hydraulic functions is performed via a very compact control block in which the control is combined to form a unit along with all the essential directional control valves and regulating valves. Such a control block is, for instance, described in the catalogue 1 987 760 507 (electronic-hydraulic lifting gear regulation for tractors) by the Applicant.
FIG. 1, which is referred to now already, illustrates the basic structure of the work hydraulics of a tractor 1 or of some other mobile implement. In accordance with FIG. 1, the tractor 1 is designed with a rear lifting gear 2 and a front lifting gear 4, the lifting cylinders 6, 8 of which can, via a control block 10, be supplied with a pressure medium of a hydraulic pump 12. In the prior art illustrated, the two lifting gears 2, 4 are configured to be single-acting (s-a), but solutions are also known in which both the front lifting gear 4 and the rear lifting gear 2 are configured to be double-acting (d-a). The control block 10 comprises electro-hydraulically actuatable directional control valves 14 that are assigned to each of the loads 6, 8 and that are controlled via an electric control device 16. The set values are, for instance, adjusted via a front operating device 18 or a rear operating device 20 which are positioned in the interior of the tractor cabin 22, or via a rear sensing device 22 positioned at the rear of the tractor, or a front sensing device (not illustrated).
For collecting the forces, pressures, speeds, and lifting gear positions occurring, a plurality of sensors, for instance, pressure sensors 26, torque sensors 28, position sensors 30, force sensors 32, or speed sensors 34, are provided at the tractor 1, the signals of which can be processed by the control device 16.
As has already been mentioned, the rear lifting gear 2 is configured to be single-acting in most known solutions, wherein the lifting cylinder 6 is extended by pressure medium being supplied via the pump 12, and the lowering is performed by the inherent weight of the rear lifting gear 2 and of the device that is mounted thereon as the case may be, e.g. a plough 36.
For drilling with a drilling machine, for instance, the rear lifting gear 2 is placed in a floating position, so that the add-on unit is in contact with the ground due to its inherent weight and passes over possible ground bumps.
With the conventional, single-action rear lifting gears, however, the bearing pressure cannot be changed actively since these lifting gears are not adapted to be operated in the field of work “pressing”. To this end, double-action rear lifting gears are required, the basic structure of which corresponds to that of the commonly used double-action front lifting gears. The double-action rear lifting gears 2 enable the lifting cylinder 6 to be controlled in the direction of “pressing”, so that an active pulling of the plough is, for instance, possible. This operating state may, for instance, also be used to lift the rear side of the tractor for changing the rear, big wheels, so that it stands on the swinging front axle and on the add-on unit actuated by the rear lifting gear, or directly on the lower hitches.
The hitherto employed pressure sensor e.g. in the front lifting gear is positioned at the bearing side and a relief pressure regulation takes place—the bearing pressure remains unknown or is not used for regulation.